TRANSMISSION AND CONDUCTION 221 
chick (unpublished) the heart of the earlier stages shows 
a distinct susceptibility gradient, the end which is to 
become the sinus end being the high end of the gradient. 
This end becomes functionally the dominant region, the 
pacemaker, and the only conceivable basis for this 
dominance is the fact that it is originally the high end 
of the gradient. Its dominance depends essentially 
upon the fact that it is able to give rise to impulses With 
a more rapid rhythm than other levels under the same 
general conditions, and this ability is unquestionably 
associated with its position in the gradient. It makes 
little difference for this conception whether we accept 
the myogenic or the neurogenic theory of the heart beat. 
If the sinus region is cooled or otherwise inhibited, 
the beat may be initiated by lower levels of the gradient, 
but the rhythm is slower. Any level of the heart is able 
to initiate a beat When sufficiently isolated from the 
sinus region, either by inhibition of the latter, by block- 
ing the passage of the beat, or by sufficient stimulation 
of the level concerned. Even reversal in direction of 
transmission of the beat is possible to a greater or less 
degree under conditions which inhibit the sinus region 
and excite the arterial end, that is, which tend to reverse 
the gradient. In all these respects the heart as a con- 
ducting path and as a gradient resembles the plate row. 
The invertebrate heart differs more or less in form in 
different groups, but is in general tubular, and, though 
it has not yet been demonstrated to be a physiological 
gradient, it behaves in general like one. 
The ascidian heart shows periodic reversal in the 
direction of transmission of the beat, the reversal 
occurring after a few or a considerable number of beats, 
